This invention relates in general to synchronizing pulse restoration circuits and particularly to apparatus for use in restoring the sync pulses in suppressed sync encrypted television signals.
A common form of television signal encryption involves suppressing the synchronizing pulses and/or inverting the video information, both on a random basis. In order to receive a usable television signal, the viewer requires a decoder unit for restoring the suppressed synchronizing pulses and for decoding the video information. Random inversion of video and random suppression of sync provides a great deal more security against signal pirating than encryption on a systematic basis. Random suppressed sync and inverted video are therefore widely used by cable operators for controlling subscriber use of their television signals.
A problem arises when decoding and processing such signals because the suppressed sync is restored by adding (or substracting) a current to the baseband video signal during the sync interval to bring the top (blank level) of the synchronizing pulses to fixed level. Thus, the restored sync pulses are uniform in height. The regular signal with unsuppressed synchronizing pulses, on the other hand, may experience sync pulse compression with consequent reduction in the sync tip level due to nonlinearities in the system headend processing, modulation and subscribeer converter IF and video processing circuits. Thus in a decoded video signal, that is one with the suppressed sync restored, the restored sync tips and normal sync tips may be at differnet levels. While synchronization is not adversely affected, the AGC circuit in the television receiver, which sets the video signal level based upon the sync tip level, causes brightness changes in the display which appear as a very disturbing flicker.
Another difficulty in cable systems results from a test procedure used by many cable operators for testing the various cables. A relatively high amplitude RF signal of known characteristics is swept across the frequency band of the cable system, either on a random or on a continous basis, and the effects thereon caused by its transit throughout the system are monitored. Departures from normal patterns indicate trouble or a system malfunction and the need for corrective action. The RF signal has both positive and negative polarity peaks which may substantially exceed the video signal in one direction and be mistaken ofr a synchronizing pulse in the other direction. An erroneous synchronizing pulse can result in loss of synchronization of the display with very noticealbe disturbances in the picture presented to the viewer. Accordingly, there is a need in the art for a sync signal restoration circuit that solves the above problems.